The invention relates to a flat screen, and in particular to an LCD screen lit by LEDs.
Known screens with liquid-crystal elements comprise an illuminating element, such as a light box, in which a plurality of fluorescent tubes, in particular CCFLs, are arranged. To render the illumination of the liquid-crystal element more uniform, light-guiding elements are arranged between the illuminating element, i.e. the tubes, and the liquid-crystal element. These light-guiding elements are films designated BEF, DBEF and PRF. For the purpose in question, the BEF film has a surface comprising a plurality of small prisms. These prisms are refractive surface elements. The DBEF film serves for changing the polarizing direction of light reflected from the rear side of the liquid-crystal element and returning it to the liquid-crystal element. The PRF film serves for polarizing the light. The use of such films has the drawback that the films absorb a portion of the light such that the integral brightness of the screen is reduced.
Further, flat screens are known which use white LEDs instead of tubes, such as CCFLs. For obtaining a relative uniform illumination and thus a relatively uniform brightness distribution across the cross-section of the flat screen, it is necessary to provide a plurality of LEDs. In known LED flat screens the LEDs are arranged at a distance to each other of approximately 1 mm. In a screen with an edge length of 40 cm×30 cm thus at least 500-1000 LEDs are provided. This entails considerable costs and a high assembly effort. Further, the large number of LEDs leads to a considerable heat development.
It is an object of the invention to provide a flat screen which can be manufactured at low costs while offering good light efficiency.